1. Field of the Invention
The present invention relates generally to the use of liquified and supercritical gases, hereafter described as dense fluids, for cleaning and sterilizing substrates. More particularly, the present invention relates to a process of using sonochemically or electrostatically energized dense fluids or dense fluid mixtures to simultaneously clean and sterilize a variety of inorganic and organic materials, including biomaterials, and to provide a method for impregnating said materials with chemical agents to provide long-term preservation and enhanced performance characteristics.
2. Description of Related Art
Conventional cleaning, sterilization, and preservation processes using hazardous organic solvents, toxic gases, radiation, and topical biocides are currently being re-evaluated due to problems with environmental pollution, toxicity, inefficiency, and/or poor performance. The use of toxic, carcinogenic, or mutagenic substances to achieve sterility have been shown to be deleterious to the environment, pose significant health threats (D. Lynch, et al, "Effects on Monkeys and Rats of Long-Term Inhalation of Ethylene Oxide: Major Findings of the NIOSH Study", AAMI, 1984), require strict control, and create hazardous waste disposal problems. Also, conventional sterilization processes may damage or alter material performance properties. For example, steam autoclaving may greatly accelerate oxide growth on titanium biomaterials (J. Lausmaa, et al, "Accelerated Oxide Growth on Titanium Implants During Autoclaving caused by Fluorine Contamination", BIOMATERIALS, Volume 6, January 1985) and must be carefully controlled.
In some cases, the cleaning or sterilizing media chemically reacts with material residues to form harmful by-products. For example, toxic by-products or residual media left in biomaterials following conventional cleaning and ethylene oxide gas sterilization have been shown to adversely impact implant performance (H. Scherer, et al, "Hazards Related to Gas Sterilized Materials, LARYNG. RHINOL. OTOL., 65, 1986). Additionally, conventional biomaterial preparation processes require a separate pre-cleaning operation prior to sterilization operations to assure complete substrate sterility. For example, in ultraviolet (UV) disinfection processes, bacterial shadowing by material structures, cavities, or other contaminants are a great concern (R. Boylan, et al, "Evaluation of an Ultraviolet Disinfection Unit", THE JOURNAL OF PROSTHETIC DENTISTRY, Volume 58, Number 5, November 1987). Since ultraviolet treatment is generally only effective on line-of-sight material sterilization applications, complex materials with intricate geometries must be scrupulously cleaned prior to UV sterilization and still may not be suitable candidates for this conventional sterilization process.
Finally, conventional long-term preservation processes are often performed as separate operations, involving immersion of, or application of topical sterilants, disinfectants, and other chemical agents. Several physical and chemical sterilization methods are used in industry. These methods include gamma radiation treatment (Ch. Baquey, et al, "Radiosterilization of Albuminated Polyester Prostheses", BIOMATERIALS, Volume 8, May 1987), ultraviolet radiation, steam autoclaving, dry heat, and toxic gas sterilization (MICROBIOLOGY, M. Peczar, et al, McGraw-Hill Publishers, 1977, pp 425-423).
Biomedical, aerospace, high energy, and high vacuum materials are fabricated from different types of materials, having various internal and external geometries. These may be assembled biomedical devices such as medical implants, valves, or artificial joints, or they may be surgical aids such as sponges, guidewires, and clips, and may be contaminated with more than one type of inorganic, organic, or biological contaminant. These highly complex materials require pre-cleaning and sterilization processes prior to use in critical environments such as the human body. Often, assembled devices must be disassembled to accommodate conventional cleaning and sterilization processes. Polymeric materials used in surgical applications, or biomaterials, must be free of organic and inorganic residues and microbiological contaminants to provide maximum biologic adhesiveness (cellular adhesion) and no biologic reactivity (Biocompatibility). These polymers must be capable of performing in contact with living tissue and body fluids. This is a highly specialized environment of great biochemical complexity. The principle medical uses of polymers include: structural materials, joint replacements, dental materials, medical devices (including tubing for transport of biofluids both inside and outside the body), adhesives, and sutures. The residual monomers, oils, plasticizers, dyes, pigments, and other additives can produce harmful side effects such as toxic chemical release through bioreaction, infection, swelling, or complete implant rejection.
Because conventional material pre-cleaning and sterilization processes are performed as independent procedures, often the sterilization procedure recontaminates the material with residues or adversely affects the physical properties and subsequent performance of the materials (J. Doundoulakis, D.M.D., "Surface Analysis of Titanium after Sterilization Role in Implant-Tissue Interface and Bioadhesion", THE JOURNAL OF PROSTHETIC DENTISTRY, Volume 58, Number 4, October 1987).
Additionally, conventional sterilization processes only deactivate biological contaminants and do not remove these deactivated residues from the material. These residues have been shown to adversely affect the performance of biomaterials following implant operations.
Finally, conventional cleaning and sterilization processes are effective only on external surfaces of composite or intricately arranged materials and provide little or no internal cleaning and sterilization capability.
Accordingly, there is a present need to provide alternate sterilization processes which are suitable for use in removing more than one type of contaminant in complex materials and sterilizing said materials prepackaged in semi-permeable membranes in one continuous process.